In U.S. Pat. No. 3,268,313 assigned to the assignee of the instant invention, a method and apparatus is shown and described for forming hollow glass fibers. In the apparatus shown therein, a plurality of orifices were provided in the bushing bottom for the passage of streams of glass from a molten glass source contained within the bushing proper. A plurality of tubes were extended into each of those orifices and were arranged substantially concentrically with the respective orifice so that each tube extended beyond the terminus of its respective orifice a fixed distance. In the claims of that patent, this distance was between 0.03 and 0.06 inches. A gas was then introduced into the tube and the anulus surrounding the tube was supplied with molten glass. This method of introducing the gas to the glass stream produced a glass fiber having a central lumen or a hollow center and the glass fibers were then gathered into continuous strand form.
While the apparatus and method described in the aforementioned U.S. patent was satisfactory in that hollow glass fibers were produced, the uniformity of the fibers was found to be somewhat less than satisfactory. It was found in operating bushings of the type described in the aforementioned U.S. patent, for example, that hollow fibers that were produced changed dramatically over time. It was found that as the bushing faceplates were subjected to continuous use, their normal tendency to warp or bow into a slight curvature from end to end occurred just as it does in a normal fiber glass bushing operation. As a result, the gas tube introducing gas into the glass stream of the bushing tip would remain stationary while the bushing tip itself would move with respect to the tube, thereby distorting the lumen of the hollow fiber being produced. This rendered the lumen of the fibers no longer concentric since the anulus formed by the bushing tip and the air tubes had become distorted from its original concentric shape. The result of this operation was the production of glass fibers which had eccentric lumens as well as erratic K values from fiber to fiber in a given strand. Strands were produced in which the fibers had extremely erratic K values and in many instances, glass strands containing large numbers of solid fibers were being produced after the bushing had been operated for an extended period.
Thus, a need exists in the art to provide a hollow fiber bushing and bushing tip arrangement which is capable of producing substantially uniform hollow glass fibers as the bushing continues to operate. By substantially uniform glass fibers is meant glass fibers in a multifiber strand which have a central lumen defining the internal diameter of the fiber and a more or less uniform outside diameter and wherein the average K value of the fibers in the strand is a reasonably high value (above 0.5) and few, if any, of the fibers contained in a given strand produced from a multi tip bushing contain solid fibers, i.e., there are less than 10 percent of the total fibers with K values below 0.5.